1. Field of the Invention
The present invention relates to an organic light emitting display and a driving method thereof, and in particular to an organic light emitting display and a driving method thereof capable of displaying an image with substantially uniform luminance.
2. Discussion of Related Art
Recently, various flat panel display devices having reduced weight and volume, which are disadvantages of cathode ray tubes, have been developed. Types of flat panel display devices include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP) and an organic light emitting display, etc.
An organic light emitting display among the flat panel display devices displays an image using organic light emitting diodes (OLEDs) that generate light using the recombination of electrons and holes. Such organic light emitting display has advantages that it has a high response speed and is driven with low power consumption.
FIG. 1 is a circuit diagram showing a pixel of an organic light emitting display. Referring to FIG. 1, the pixel 4 of the organic light emitting display includes a pixel circuit 2 coupled to an organic light emitting diode OLED, a data line Dm, and a scan line Sn to control the organic light emitting diode OLED.
An anode electrode of the organic light emitting diode OLED is coupled to the pixel circuit 2 and a cathode electrode of the organic light emitting diode OLED is coupled to a second power supply ELVSS. The organic light emitting diode OLED is light emitted at luminance corresponding to current supplied from the pixel circuit 2.
The pixel circuit 2 controls the amount of current supplied to the organic light emitting diode OLED corresponding to a data signal supplied to the data line Dm when a scan signal is supplied to the scan line Sn.
To this end, the pixel circuit 2 includes a second transistor M2 coupled between a first power supply ELVDD and the organic light emitting diode OLED; a first transistor M1 coupled between the second transistor M2, the data line Dm, and the scan line Sn; and a storage capacitor Cst coupled between a first electrode and a gate electrode of the second transistor M2.
A gate electrode of the first transistor M1 is coupled to the scan line Sn and a first electrode of the first transistor M1 is coupled to the data line Dm. A second electrode of the first transistor M1 is coupled to one terminal of the storage capacitor Cst.
Herein, the first electrode is one of a source electrode and a drain electrode and the second electrode is the other one of the source electrode and the drain electrode. For example, if the first electrode is the source electrode, the second electrode is the drain electrode. The first transistor M1 coupled to the scan line Sn and the data line Dm is turned on when the scan signal is supplied from the scan line Sn to supply the data signal supplied from the data line Dm to the storage capacitor Cst. At this time, the storage capacitor Cst charges voltages corresponding to the data signal.
The gate electrode of the second transistor M2 is coupled to one terminal of the storage capacitor Cst and the first electrode of the second transistor M2 is coupled to the other terminal of the storage capacitor Cst and the first power supply ELVDD. The second electrode of the second transistor M2 is coupled to the anode electrode of the organic light emitting diode OLED.
The second transistor M2 controls the amount of current flowing from the first power supply ELVDD to the second power supply ELVSS via the organic light emitting diode OLED, where the amount of current corresponds to a voltage value stored in the storage capacitor Cst. At this time, the organic light emitting diode OLED generates light corresponding to the amount of current supplied from the second transistor M2.
However, there is a problem that such an organic light emitting display cannot display an image with desired luminance due to the efficiency change according to the degradation of the organic light emitting diode OLED.
In practice, the organic light emitting diode OLED is degraded as time elapses so that light with gradually reduced luminance is generated. Also, the conventional organic light emitting display has a problem in that the image with uniform luminance is not displayed due to the non-uniformity of the threshold voltage/mobility of the driving transistor M2 included in the pixels 4.